Syntax check results should be shown by underlining

Description

Instead of having its own tab in the bottom pane, syntax errors should be reported by underlining, with further explanations (the type of error) in tooltips.

This will spare us one tab in the bottom pane, making it (depending on the circumstances) possibly unnecessary, and thereby freeing vertical screen space that can be used to a larger part of the files that are in the editor.

For illustration, I have attached an example of how this looks like in MonoDevelop?.